台灣位於太平洋西側、歐亞大陸東側的熱帶與副熱帶季風地區，春夏季轉換時、梅雨鋒面、夏季的颱風和午後熱對流常常帶來豪大雨，在加上台灣地形陡峭，容易造成水災、土石流，因此準確預估降水對防災有極大的幫助。雷達回波與降雨率的關係廣泛被用來估計降水的方法之一，可以用在大範圍降水估計;缺點則是空間上的雨滴粒徑變化太大，相同的回波值對應的降雨率範圍很廣， 兩者並非一對一的關係，因此了解雨滴粒徑分佈的特性將有助於改善降水估計。不同的地區、不同的降水型態，都有可能造成雨滴粒徑分佈不同，以及分析比較降水積分參數有助於了解不同的降水特性。

本研究使用的資料來源為中央大學Joss-Waldvogel disdrometer(JWD)觀測資料與中央氣象局(CWB)三維雷達回波合成資料(QPESUMS)，統計的區間為2005年1月至2014年12月間。由標準化的gamma分布顯示,平均直徑Dm (Mass-weighted average diameter)於夏季有最高值，而平均Nw(normalized intercept parameter)最高值則出現在冬季; 透過雷達回波在高度上的變化得知垂直發展影響DSD (drop size distribution)在不同季節的結果。此外，台灣北部降雨率多集中於20mm/hr以下(層狀性降水)，本篇研究使用雷達回波區分對流性與層狀性降雨，移除層狀性降水主導的因素。所有季節的層狀降雨皆有相近的DSD分布結構，但對流降雨的DSD則是偏向海洋型對流; 平均Dm在對流降雨系統有較高值，而平均log10Nw在層狀系統內有較高值，透過 Contoured frequency by altitude diagrams (CFADs)發現垂直發展主導DSD的變異性。

Drop size distribution (DSD) is a metric widely used in meteorology and hydrology. Taiwan is located in a subtropical monsoon area in the west Pacific Ocean off the coast of East Asia. Enormous quantities of rainfall during the transition season often cause flooding and mudslides. Accurate rainfall prediction can help to alleviate the effects of such rainfall events. DSD varies with regard to the type of rain as well as its spatial distribution. Radar reflectivity-rate of rainfall (Z-R) relations are strongly dependent on DSD variations, which means that it is important to analyze the DSD in various seasons as well as in various types of rain.

Between January 2005 and December 2014, DSD data was collected using a Joss-Waldvogel Disdrometer (JWD) to analyze variations in the Gamma parameters of raindrop spectra at NCU (24°58'6"N 121°11'27"E). The normalized Gamma distribution of DSD revealed that the highest mean Dm (Mass-Weighted Average Diameter) values were in summer, whereas the highest mean log10Nw (normalized intercept parameter) values were in winter. Vertical structures detected in radar reflectivity profiles dominate the results of seasonal DSD. Furthermore, most of the rain falling at less than 20 mm/hr (stratiform precipitation) occurs in Northern Taiwan. In this study, we used radar reflectivity to differentiate between convective and stratiform systems. It was discovered that the mean Dm value is higher in convective systems, whereas the mean log10Nw value is higher in stratiform systems. The structure of DSD in stratiform systems remains constant in all seasons; however, convection is similar to maritime type. Contoured Frequency by Altitude Diagrams (CFADs) revealed that vertical structures dominate DSD in various types of precipitation.

Beard KV, Ochs HT. 1993. Warm-rain initiation: an overview of microphysical mechanisms. J. Appl. Meteorol. 32:608–25
Bringi, V. N., and V. Chandrasekar, J. Hubbert, E. Gorgucci, W. Randeu, and M. Schoenhuber,2003: Raindrop size distribution in different climateregimes from disdrometer and dual-polarized radar analysis. J. Atmos. Sci., 60, 354–365.
Chaing, Y.-C, 2010: The characteristic of drop size distribution of Mei-Yu season in 2009. Master Thesis, National Central University, 107 pages. (in Chinese)
Chang, W.-W, 2002: Using disdrometer to analyze the Drop size distribution (Typhoon Nari). Master Thesis, National Central University, 95 pages. (in Chinese)
——, T.-C. Wang, and P.-L. Lin, 2009. Characteristics of the raindrop size distribution and drop shape relation in Typhoon systems in the western Pacific from the 2D Video Disdrometer and NCU C-band polarimetric radar. J. Atmos. Oceanic Tech., 26, 1973-1993
Chen Baojun, Yang Jun, and Pu Jianping, 2013: Statistical characteristics of raindrop size distribution in the Mei-yu season observed in eastern China. J. Meteor. Soc. Japan. Ser. II, 91, 215–227.
Chen, C.-S., and Y.-L. Chen, 2003: The rainfall characteristics of Taiwan. Mon. Wea. Rev., 131, 1323–1341
Chen, Y.-C., 2013: Comparison in the frontal system of strong precipitation of drop size distribution in northern Taiwan. Master Thesis, National Central University, 111 pages. (in Chinese)
Chien, C.-L, 2006: The characteristic of drop size distribution in different season and precipitation types in northern Taiwan. Master Thesis, National Central University, 119 pages. (in Chinese)
Gamache, J. F., and R. A. Houze, Jr., Mesoscale air motions associated with a tropical squall line, Mon. Weather Rev., 110,118-135, 1982
Glickman, T. S., (Ed.), Glossary of Meteorology, Am. Meteorol. Soc., 855 pp, 2000
Gunn, K. L. S., and G. D. Kinzer, 1949: The terminal velocity of fall for water droplets in stagnant air. Meteorology, 6, 243–251.
Houze, R. A., Jr.,1997: Stratiform precipitation in regions of convection: A meteorological paradox? Bull. Amer. Meteor. Soc., 78, 2179–2196.
Hsu, Y.-C, 2005: The characteristic of Drop size distribution of northern Taiwan and rainfall estimation. Master Thesis, National Central University, 89 pages. (in Chinese)
Islam, T., M. A. Rico-Ramirez, M. Thurai, and D. Han, 2012: Characteristics of raindrop spectra as normalized gamma distribution from a Joss-Waldvogel disdrometer. Atmos. Res., 108, 5773.
Jayalakshmi, J., Reddy, K.K., 2014. Raindrop size distributions of south west and north east monsoon heavy precipitations observed over Kadapa (14o 4′ N, 78o 82′ E), a semiarid region of India. Curr. Sci. 107 (8), 1312–1320
Kozu, T., and K. Nakamura, 1991: Rainfall parameter estimation from dual-radar measurements combining reflectivity profile and path integrated attenuation. J. Atmos. Oceanic Technol., 8, 259–270
——, K. K, Reddy, S.Mori, M. Thurai,J.T. Ong, D. N.Rao,and T. Shimomai, 2006: Seasonal and diurnal variations of raindrop size distribution Asian monsoon region, J. Meteon Soc. krpan, 84A, 195-209
Krishna, M., K. K, Reddy, B. K., Seela, R. Shirooka, P.-L. Lin., C.-J. Pan., 2016: Raindrop size distribution of easterly and westerly monsoon precipitation observed over Palau islands in the Western Pacific Ocean. Atmospheric Research, 174–175, 41–51
Lau, K. M. and Wu, H. T., 2003: Warm rain processes over tropical oceans and climate implications, Geophys. Res. Lett., 30, doi:10.1029/2003GL018 567
Lu, Y.-C., 2012: Observation of rain drop size distribution during the invaded time of typhoon Fanapi in Taiwan. Master Thesis, National Central University, 85 pages. (in Chinese)
Maki, M., T. D. Keenan, Y. Sasaki, and K. Nakamura, 2001: Characteristics of the raindrop size distribution in tropical continental squall lines observed in Darwin, Australia. J. Appl. Meteor., 40, 1393-1412
Mao, Y.-Y, 2007: The characteristic of rain drop size distribution between convective and stratiform precipitation in northern Taiwan. Master Thesis, National Central University, 101 pages (in Chinese)
Marshall, J. S., and W. M. Palmer, 1948: The distribution of raindrops with size. J.Meteor., 5, 154-166
Marzuki, M., Hashiguchi, H., Yamamoto, M.K., Mori, S., Yamanaka, M.D., 2013c. Regional variability of raindrop size distribution over Indonesia. Ann. Geophys. 31, 1941–1948.
Sauvageot, H., and J.-P. Lacaux, 1995: The shape of averaged raindrop size distributions. J. Atmos. Sci., 52, 1070–1083.
Schumacher, C., and R. A. Houze, The TRMM precipitation radar view of shallow, isolated rain, J. Appl. Meteorol., 42, 1519 – 1524, 2003.
Steiner, M., R. A. Houze Jr., and S. E. Yuter, 1995: Climatological characterization of three-dimensional storm structure from operational radar and rain gauge data. J. Appl. Meteor., 34, 1978– 2007.
Tenório, R.S., da Silva, Cristina, Moraes, M., Sauvageot, H., 2012. Raindrop size distribution and radar parameters in coastal tropical rain systems of northeastern Brazil. J. Appl. Meteorol. Climatol. 51, 1960–1970.
Testud, J., S. Oury, P. Amayenc, and R. A. Black, 2001: The concept of ‘‘normalized’’ distributions to describe raindrop spectra: A tool for cloud physics and cloud remote sensing. J. Appl. Meteor.,40, 1118–1140.
Thompson, J. E., S. A. Rutledge, B. Dolan, and M. Thurai, 2015: Drop size distributions and radar observations of convective and stratiform rain over the equatorial Indian and west Pacific Oceans. J. Atmos. Sci., 72, 4091–4125
Tokay, A., and D. A. Short, 1996: Evidence from tropical raindrop spectra of the origin of rain from stratiform versus convective clouds. J. Appl. Meteor., 35, 355–371
——, ——, C. R. Williams, W. L. Ecklund, and K. S. Gage, 1999: Tropical rainfall associated with convective and stratiform clouds:Intercomparison of disdrometer and profiler measurements. J. Appl.Meteor., 38, 302–320.
Ulbrich, C. W., 1983: Natural variations in the analytical form of the raindrop size distribution. J. Climate Appl. Meteor., 22, 1764–1775
——, and D. Atlas, 1984: Assessment of the contribution of differential polarization to improved rainfall measurements. Radio Sci., 19, 49–57.
——, ——, 2007: Microphysics of raindrop size spectra: Tropical continental and maritime storms. J. Appl. Meteor. Climatol., 46, 1777–1791.
Waldvogel, A., 1974: The No jump of raindrop spectra. J. Atmos. Sci., 31, 1067–1078
Wu, H.-S, 2006: By using distrometer to analyze the microphysical characteristic in different precipitation types. Master, Thesis of National Central University, 101 pages (in Chinese)
Zhang, J., K. Howard, and J. J. Gourley, 2005: Constructing threedimensional multiple-radar reflectivity mosaics: Examples of convective storms and stratiform rain echoes. J. Atmos. Oceanic Technol., 22, 30–42.